Production of mono-crystal semiconductor bodies



April 25, 1961 J. L. PARMEE 2,981,687

PRODUCTION OF MONO-CRYSTAL. SEMI-CONDUCTOR BODIES Filed March 26, 1959 //VV/V7'0/'-? JOHN LLEWfLL/N E/QFMEE QrTOFPA/EY PRODUCTION or MONO-CRYSTAL SEMI- v CONDUCTOR BODIES John Llewellin Parmee, Rugby, England, assignor to The L British Thomson-Houston Company Limited, London,

England, a British company Filed Mar. 26, 1959,Ser. No. 802,131 Claims priority, application Great Britain Apr. 3, 1958 2 Claims. to]. 2S2 62.3

This invention relates to the production of bodies of mono-crystal semi-conductors, such as germanium and silicon, containing a desired'impurity activator.

The usual process of producing mono-crystal bodies of germanium and silicon is by crystal pulling. In this process a seed crystal ofthe semi-conductor is' introduced into a molten region in the centre of a mass of the purified material contained in a: crucible of comparatively large diameter, and the seed crystal is gradually withdrawn from themolten region, the adherent material solidifying and adopting the crystal structure of the seed; If the molten material is of N- or P-type conductivity, the emergent mono-crystal ingot is of the corresponding conductivity type. r

In the process according to the invention a rod or ingot of substantially pure semi-conductor material located in a vertically-disposed closely surrounding tube of quartz and maintained in a protective atmosphere is locally melted at its upper end in contact with a mass of, or containing a desired impurity activator; A. seed crystal of the semi-conductor is introduced into contact with the surface of the molten mass, and'the crystal is gradually withdrawn from the body whereby the adherent material withits impurity activator progressively solidifies behind and acquires the mono-crystal structure of the seed, the zone of the ingot in contact with the mass and the mass being kept in molten condition tomaintain the supply of semi-conductor material to be solidified behind the seed.

The molten zone can conveniently be established and maintained by a high frequency heating coil surrounding the quartz tube, and the diameter of the frozen monocrystal caube keptuniform by feeding the tube in the direction of the withdrawal at a predetermined constant relatively velocity. r

The body containing the impurity activator may consist of the pure activator, or of an activator-rich alloy of the semi-conductor.

By accurately calculating and determining the size of the body of, or containing, the impurity activator for a given volume of molten mass, uniform resistivity can be maintained in the frozen mono-crystal.

Furthermore, since the ingot isprogressively melted, the molten zone is only in contact with the quartz for a short period, and the amount of contamination from the wall of the quartz tube is very much less than is the case when a crystal of the same total mass is pulled from a molten mass continuously in contact with the wall of a crucible in which it is contained. By the process, crystals of large and uniform diameter are able to be grown.

The accompanying drawing shows, in cross-section, a form of apparatus which may be used in carrying out the invention.

An ingot of substantially pure semi-conductor material, that is material containing less than 0.001 part per million of impurities, is located in a tubular container 2. The material may be germanium or silicon; when germanium is used the container 2 may consist of fused quartz or graphite; for silicon quartz only should be employed for molten, may be employed. The gas introduced through 2,981,687 Patented Apr, 25, 1961 ice the container. The container 2. is enclosed in. an outer shield 3 into which, during the operation of the process,

a non-oxidizing gas is introduced through opening 4. A suitable non-oxidizing gas is argon; but any such gas as 'will not react with the semi-conductor .material, when opening 4 flows around the container and escapes by way of an aperture 5 in the cover 6 of the container.

Container 2 is supported inthe shield by means of a stand in the form of a perforated quartz tube 7. Cover 6 also contains two viewing apertures 8 above which are mounted prisms 9 through. which the progress of the process taking place within the shield may be inspected. A chuck 10 in which a seed crystal 11 is held passes through the aperture 5 in the cover 6 and may be raised and lowered by suitable mechanism, not shown. It is also desirable for the chuck to be arranged for rotation about its longitudinal axis, asindicated by the arrow 10'.

For the purpose of applying localised heating to the ingot 1,. and a pellet containing the impurity activator a source of high frequency oscillation by way of a transformer, and in order that the localised zone in which heating is eifected may be caused to travel along the ingot,.

relative movement of the coil and container is provided for, preferably by mounting coil 12 so that it may be moved vertically. r

- In carrying out the process, ingot 1 in its container 2, which is preferably a close fit around the ingot, is introduced into the shield 3 after removal of the cover therefrom, the cover being thereafter replaced in position. A pellet of an alloy of the semi-conductormaterial with a suitable activator is then placedon the uppersurface of the ingot, and chuck 10 having a seed crystal of the semiconductor is then introduced through aperture 5 and lowered into contact with the pellet. Radio frequency oscillations are then supplied to the coil 12, which. is positioned in the'vicinity of the upper surface of the ingot 1.

three minutes, a localised zone, including the upper por-.

tion of the ingot and the pellet in contact therewith, is caused to become molten, the chuck is then adjusted so that the seed crystal is in contact with the mass of molten material and the rotation of the chuck and its slow withdrawal upwardly from the molten mass is initiated.

The melting of the upper layer of the ingot and the pellet in contact therewith causes the formation of a molten region of semi-conductor material in which the impurity contained in the pellet is uniformly distributed. As the chuck containing the seed crystal moves away from the molten region, the semi-conductor in contact with the crystal acquires the mono-crystal structure of the seed and solidifies, drawing with -it an adherent body 13 of mono-crystal semi-conductor material containing a desired concentration of the impurity activator. The lower portion of the body 13 remains in contact with the molten region 14 which is maintained by the continuing supply of radio frequency energy to the coil 12. Coil 12 is lowered as the chuck is raised until substantially the whole of the ingot is formed into a mono-crystalline body of semi-conductor material. The mass of molten material finally remaining at the bottom of the container 2 will then include all the impurity activator which has not been acquired by the mono-crystal body.

The amount of impurity activator remaining in the 3 solidfied mono-crytal body will be uniformly dispersed throughout the body as a result of the condition under which the body has been produced, and the number of atoms per cc. of the activator in the body will depend on the amount of activator present in the pellet and the segregation constant of the impurity activator.

Example I.T produce a 50 ohm cm. n-type silicon ingot having a uniform resistivity profile along its length 50; ohm cm. n-type silicon corresponds to an activator concentration of atoms/cc.

' Using antimony as the activator (segregation coefiicient -=.04),. the activator concentration in the molten zone =25 X 10 atoms/cc.

Thus for a 4 cm. diameter ingot and a 1.5 cm. zone length approximately 4 mgm. of 0.1% Sb/Si alloy is rerequired,

Example ll. To produce a 50 ohm cm. p-type silicon ini got having a uniform resistivity profile along its length 50 ohm cm. p-type silicon corresponds to an activator concentration of 2.5 x10 atoms/cc.

, Using aluminum as the activator (segregation coefiicient='.004) the activator concentration in the molten zone=6 l0 atoms/cc.

Thus for a 4 cm. diameter ingot and a 1.5 cm. zone length approximately 21 mgm. of 0.1% Al/Si alloy is required.

The process may be applied to both germanium and silicon, but is of greater application to the production of mono-crystal silicon in view of the higher temperature which is required to melt silicon (142l C.). The dif-,

ficulty of reducing contamination of the silicon at this high temperature from the container, quartz tube or crucible, is considerable; by the present process the contamination is reduced as above described.

The relative movements between the heating coil and the quartz tube required to maintain the desired molten zone in the ingot and to allow for the freezing of the material behind the seed crystal may be produced in any suitable manner.

What I claim is:

1. Process of preparing a body of mono-crystal silicon containing an impurity activator selected from the group consisting of aluminum and antimony which consists in locating an ingot of substantially pure silicon in a vertically disposed closely surrounding tube of quartz, maintaining an atmosphere of non-oxidizing gas around said tube, locating a mass of silicon containing said impurity activator in contact with the upper surface of said ingot, heating a zone including the upper end of said ingot and 4 i said mass to cause melting only of said mass and said upper end of said ingot, introducing a seed crystal of silicon into contact with the molten sunface of said mass,

and withdrawing said seed crystal from said heated zone' ally of the ingot with the withdrawal of moltenmateriaL with the withdrawal of the seed crystal, the heating of the ingot being maintained solely in said zone to assure that only the small upper end of the ingot necessary for the continued supply of molten material for solidification behind said seed crystal is at anytime in molten state contact with said tube.

2. Process of preparing a body of mono-crystal silicon containing an impurity activator selected from the group consisting of aluminum and antimony, which consists in locating an ingot of substantially pure silicon in a vertically disposed closely surrounding quartz tube, maintaining a non-oxidizing atmosphere around said tube and ingot, locating a pellet of silicon containing said activator in contact with the upper surface of said ingot, inductively heating a zone including said pellet and the upper end of said ingot so as to cause melting of said pellet and l0- calised melting of said ingot, introducing a seed crystal of silicon into the molten mass, and withdrawing said seed crystal from said zone whereby material adherent,

said seed crystal, and heating the ingot progressively longi-.

tudinally thereof as the upper end of the ingot moves corresponding'ly longitudinally of the ingot with the withdrawal of molten material with the withdrawal of the seed crystal, the heating of the ingot being maintained solely in said zone to assure that only the upper end of the ingot necessary for the continuance of the supply of molten material adherent to, and for solidification behind the body withdrawn from said tube, and for the maintenance of a molten mass containing a predetermined concentration of said impurity activator is at any time in molten state contact with said tube.

References Cited in the file of this patent UNITED STATES PATENTS Horn Sept. 15, 1959 

2. PROCESS OF PREPARING A BODY OF MONO-CRYSTAL SILICON CONTAINING AN IMPURITY ACTIVATOR SELECTED FROM THE GROUP CONSISTING OF ALUMINUM AND ANTIMONY, WHICH CONSISTS IN LOCATING AN INGOT OF SUBSTANTIALLY PURE SILICON IN A VERTICALLY DISPOSED CLOSELY SURROUNDING QUARTZ TUBE, MAINTAINING A NON-OXIDIZING ATMOSPHERE AROUND SAID TUBE AND INGOT, LOCATING A PELLET OF SILICON CONTAINING SAID ACTIVATOR IN CONTACT WITH THE UPPER SURFACE OF SAID INGOT, INDUCTIVELY HEATING A ZONE INCLUDING SAID PELLET AND THE UPPER END OF SAID INGOT SO AS TO CAUSE MELTING OF SAID PELLET AND LOCALISED MELTING OF SAID INGOT, INTRODUCING A SEED CRYSTAL OF SILICON INTO THE MOLTEN MASS, AND WITHDRAWING SAID SEED CRYSTAL FROM SAID ZONE WHEREBY MATERIAL ADHERENT TO SAID SEED CRYSTAL AND CONTAINING SAID IMPURITY ACTIVATOR SOLIDIFIES WHILST ACQUIRING THE MONO-CRYSTAL STRUCTURE OF SAID SEED CRYSTAL, AND HEATING THE INGOT PROGRESSIVELY LONGITUDINALLY THEREOF AS THE UPPER END OF THE INGOT MOVES CORRESPONDINGLY LONGITUDINALLY OF THE INGOT WITH THE WITHDRAWAL OF MOLTEN MATERIAL WITH THE WITHDRAWAL OF THE SEED CRYSTAL, THE HEATING OF THE INGOT BEING MAINTAINED SOLELY IN SAID ZONE TO ASSURE THAT ONLY THE UPPER END OF THE INGOT NECESSARY FOR THE CONTINUANCE OF THE SUPPLY OF MOLTEN MATERIAL ADHERENT TO, AND FOR SOLIDIFICATION BEHIND THE BODY WITHDRAWN FROM SAID TUBE, AND FOR THE MAINTENANCE OF A MOLTEN MASS CONTAINING A PREDETERMINED CONCENTRATION OF SAID ACTIVATOR IS AT ANY TIME IN MOLTEN STATE CONTACT WITH SAID TUBE. 